1. Field of the Invention
The present invention relates to a swirl control system for, so called, lean-burn engines. Specifically to a swirl control system for lean-burn engines, utilizing composite fuel blended with at least two different liquid fuels, such as gasoline and alcohol.
2. Description of the Prior Disclosure
Recently, there have been proposed and developed various lean-burn engines employing a swirl control system so as to assure optimal combustion in accordance with the different operating states of the engine, for example the magnitude of engine load and/or engine speed. As is generally known, swirl occurring in each combustion chamber of a multi-cylinder lean-burn engine prevents deterioration in combustion efficiency due to a leaner air-fuel gas mixture having a relatively high proportion of air and a low proportion of fuel, i.e., a leaner gas mixture than that of the stoichiometric air-fuel ratio or due to an excessive amount of exhaust-gas recirculated into the combustion chambers through an exhaust-gas recirculation (EGR) system. However, if the intensity of the swirl is excessively increased during high engine revolutions, the amount of intake air entering the combustion chambers is lowered due to excessively increased flow resistance of swirling air introduced into an intake port behind the intake valve of the engine and a lowering in density of the introduced air, thereby resulting in lower engine output. For this reason, a swirl control system is required for adjusting the intensity of swirl in response to the engine operating state.
Such conventional swirl control systems have been disclosed in Japanese Patent First Publications (Tokkai) Showa 61-192811, 61-272418 and 62-63128, and Japanese Utility Model First Publications (Jikkai) Showa 61-151037 and 62-79936. One such conventional swirl control system for internal combustion engines includes an intake air flow passage divided, upstream of an intake port, into a main intake air passage and an auxiliary intake air passage defining a relatively narrow flow passage helically shaped so as to facilitate swirling motion. The prior art swirl control system also includes a swirl control valve disposed in the main intake air passage for controlling the intensity of swirl occurring in the combustion chamber in such a manner as to open or close the main intake air passage. For example, when the engine is operated in a low or medium load state, the swirl control valve is closed, thereby resulting in a strong swirl in the combustion chamber. Such strong swirl improves combustion in lean-burn engines during low or medium engine load and consequently improves the specific fuel consumption of the engine.
When the engine is in a high load state, the swirl control valve is opened and as a result a charging efficiency of intake air is enhanced to assure higher engine output.
Just after engine starting, when a very rich air-fuel mixture is required for optimal combustion, the swirl control valve is opened so as to prevent fuel flowing through an inner wall of the combustion chamber from adhering to the ignition plug.
However, since previously described conventional swirl control systems are so tuned as to be suitable for internal combustion engines utilizing only gasoline as liquid fuel, such a swirl control system would not be suitable for internal combustion engines utilizing composite fuel blended with at least two different liquid fuels, for example methanol-gasoline blends, ethanol-gasoline blends or the like, because such a composite fuel exhibits specific fuel consumption characteristics considerably different from gasoline. The specific fuel consumption is dependent on a mix proportion between two different liquid fuels as mentioned above. Therefore, if the swirl control system so tuned as to be suitable for lean-burn engines utilizing only gasoline is used for lean-burn engines possibly utilizing various mix proportions of composite fuels, at low or medium engine load conditions, an optimal lean-burn cannot be obtained, thereby resulting in unstable combustion in the combustion chambers. As a result, specific fuel consumption, emission performance, and driveability performance of the engine, such as startability and warm-up performance are deteriorated. In order to overcome the problems of the prior systems and increase the performance efficiency of swirl control systems, there is herein proposed an improved swirl control system which can provide optimal lean-burn in response to the specific mix proportion of the composite fuel used for a lean-burn engine.